1. Field of the Invention
This invention relates to a novel and improved integrated sputtering apparatus for use in triode sputtering and, more in particular, to an integrated sputtering means having an ion target of a selected material positioned interior to a thin passageway in a housing and magnetic means which establishes a controlled magnetic field of flux to encapsulate both electrons and plasma, thereby increasing efficiency of sputtering of selected material from the ion target surface. Triode sputtering apparatus includes an electron emitter and an electron collector to produce, in an evacuated enclosure, a controlled flow of electrons which collide with an ionizable gas within the evacuated enclosure thereby, forming a gas plasma. Ions of gas plasma are attracted toward and impinge into an ion target surface formed of a selected material. The collision between the plasma ions and ion target surface eject or sputter atoms of the selected ion target material therefrom producing a vapor of ion target material within a certain portion of the evacuated enclosure. A substrate is positioned in the evacuated enclosure and within the ion target material vapor. Atoms of the ion target material adhere to a surface of the substrate, forming a thin film of atoms of deposited ion target material thereon.
2. Disclosure of the Prior Art
The technique of physical sputtering and use of the sputtering process for deposition of thin films on a substrate surface are well known in the prior art. In depth, disclosure and description of these phenomena are described in The Handbook of Thin Film Technology, published by McGraw-Hill, 1970, at pages 3-1 to 3-38 in a section entitled "Chapter 3, The Nature of Physical Sputtering" by Gottfried K. Wehner and Gerald S. Anderson and at pages 4-1 to 4-44 in a section entitled "Chapter 4, Application of Sputtering to the Deposition of Films" by Leon Maissel.
A number of issued United States Patents and publications disclose various methods and apparatus for sputtering, and the following are deemed relevant as known prior art to this invention.
U.S. Pat. No. 3,393,142 issued to R. M. Moseson on July 16, 1968, discloses a cathode sputter apparatus with plasma confining means which utilizes a triode sputtering apparatus having means for establishing an ion plasma adjacent an ion target. The electron releasing cathode includes apertures and nozzles to impart a desired configuration to the electrons. The electrons collide with an ionizable gas forming the ion plasma. In the absence of a magnetic field, the ion plasma tends to diverge in a flat wedge-shaped configuration near the anode. An electromagnetic coil, located to the exterior of the evacuated enclosure containing the triode sputtering apparatus, is positioned to establish parallel field lines at the end of the ion plasma which, in the absence of a magnetic field, would tend to diverge as described. The magnetic field has longitudinal and unidirectional field lines extending primarily through an open space located between the substrate and opposed parallel ion target. The substrate and target are positioned substantially parallel to the axis of the ion plasma sheet located therebetween.
U.S. Pat. No. 3,487,000 issued to Hajzak on Dec. 30, 1969, discloses an evacuable, rectangular, box-like ion plasma confining enclosure located within an evacuable chamber for confining the ion plasma. Electrons from an electron source are emitted into the box-like chamber. An ionizable gas is admitted into the enclosure for forming the ion plasma and a target is supported within the box-like chamber on a target support assembly. A substrate is mounted over a rectangular aperture formed in one wall of the enclosure and receives the sputtered material. The box-like enclosure is electrically isolated and has an electrical charge formed thereon from the ion plasma which reduces electron drain from the plasma.
U.S. Pat. No. 3,516,919 issued to F. Gaydou et al on June 23, 1970, teaches the use of an external magnetic field in combination with cathode sputtering apparatus. The ion target is located in a second chamber or anode cavity. The vacuum is maintained between 10.sup.-4 to 10.sup.-3 torr. The ionizable gas is introduced in the vicinity of the thermionic cathode. The external magnetic field is an electromagnet which produces a magnetic field of a few hundred gauss with the magnetic lines of force parallel to the axis of the plasma stream. The plasma stream is relatively circular in cross-section. The magnetic field acts to confine the path of the plasma so sputtering can be accomplished at higher rates. The substrates are mounted on an arcuate shaped support assembly facing toward the ion target assembly, which assembly in turn is surrounded by an anode.
An article appearing in the February 1971 issue of Research/Development at pages 40 to 44, inclusive, entitled "Crossed Field Discharge Device For High Rate Sputtering" by James R. Mullaly, discloses a magnetron sputtering apparatus and describes the known prior art. In the magnetron sputtering apparatus, the arcuate shaped cathode functions as an electron emitter and target. A ring anode is located around one edge of the cathode. During the sputtering process, selected portions of the cathode are intensely deteriorated or eroded as material is sputtered therefrom, producing a ring source of sputtered material. A magnetic field, described either as a quadrupole field or single-cusp magnetic mirror, is produced by electromagnetic coils located external to the apparatus. The magnetic lines of force form a "V-shaped" pattern commencing at one edge of a substrate and extending into two directions, one toward the cathode and one toward and passing through the substrate. The magnetic flux density is in the order of 200 gauss or less. The magnetic field, produced by the external electromagnets, and electric field between the anode and cathode are static, forcing the electrons into long cycloidal paths. An ionizable gas is discharged into the evacuated chamber. The apparatus operates typically in a vacuum of 5 .times. 10.sup.-3 torr.
U.S. Pat. No. 3,654,123 issued to Hajzak on Apr. 4, 1972, teaches the use of a triode sputtering apparatus in combination with an external electromagnetic flux means having magnetic coils, a flux strap, curved flux distributing plates mechanically connected to the flux strap and signal generating means. The curved flux distributing plates generate an essentially uniform flux or B-field within the evacuated enclosure. The uniform flux tends to condense or confine the plasma to the spacial region of the target. The target is supported and positioned, without confinement, between the cathode and anode. The flux strap confines the flux outside the evacuated enclosure within the strap. Electrons emitted from the cathode are shaped by an electron deflector into an elongated rectangle to achieved sheet-like emitted electron flow and plasma. In the absence of the B-field, a cone-like plasma would be formed. The signal generating means periodically cycles the B-field and varies its strength and orientation.
U.S. Pat. No. 3,669,860 issued to Knowles et al on June 13, 1972, discloses a diode sputtering apparatus wherein the cathode functions as the electron emitter and target. A magnetic field produced by a plurality of externally located electromagnets extends transversely through the space between the cathode and substrate to deflect electrons emitted from the cathode transversely clear of the substrate. The magnetic field is electrically rotated about a path or axis extending between the cathode and substrate being coated.
U.S. Pat. No. 3,878,085 issued to Corbani on Apr. 15, 1975, discloses a magnetron or cathode sputtering apparatus wherein the face of the cathode to be sputtered is formed into closed annular shaped configuration. Magnetic means are located adjacent to the cathode at a side opposite the cathode face. The magnetic means include a pair of magnetic pole pieces, at least one of which is elongated, which form arcuate shaped magnetic lines of force. The magnetic lines of force form a tunnel-like path wherein all side walls are magnetic lines of force and the bottom is the cathode face. Since the cathode face is formed into a closed or annular shaped configuration, the tunnel-like path forms a ring of arcuate shaped magnetic lines of force which tend to trap charged particles within the ring and against the cathode face. The charged particles then whirl around the inside of the ring adjacent to the cathode face. Thus, the magnetic field is formed into a closed loop or "racetrack" configuration, the result of which is to increase sputtering activity.
U.S. Pat. No. 3,956,093 issued to McLeod on May 11, 1976, discloses a planar magnetron sputtering apparatus which includes a ring-shaped closed loop magnetic field around the plate of a cathode face being sputtered. A second variable magnetic field is produced from an AC signal controlled electromagnetic field device producing lines of force which are generally normal to the surface of the cathode plate to vary the erosion depth on the cathode plate throughout the erosion region.
In order to better appreciate the teachings of the present invention, the operating characteristics and differences of the cathode sputtering apparatus and the triode sputtering apparatus will now be considered.
Magnetron-type sputtering sources, of which Corbani U.S. Pat. No. 3,878,085 and McLeod U.S. Pat. No. 3,956,093 are typical, utilize the cathode both as a target and as a secondary source of electrons. Electrons generated at the target arise from secondary emission and photoemission. The quantity of generated electrons is determined by a number of factors, such as, for example, target material, target voltage, chamber pressure and plasma ion species. During the sputtering operation, a predetermined quantity of target-generated electrons results for a given fixed target current density (all other operational parameters constant) at known different targe voltage level for each different target material. For example, a typical magnetron source operating at a fixed target current density of about 50 millamperes per square centimeter would require a target voltage level of about -300 volts for a niobium target and about -700 volts for a gold target. Differing plasma impedance levels exist for different target materials at a given operating target voltage. The operating voltage is a dependent parameter based on source design and operating conditions. Each are inherent qualities of a magnetron source.
Relying on a target to supply a limited quantity of electrons to the plasma discharge has a further restriction in that such sources cannot operate a high current densities below vacuum pressures of approximately 2 .times. 10.sup.-3 torr. Below this pressure, there is an insufficient quantity of electrons generated and an insufficient quantity of ionized gas molecules present to sustain continuous plasma operation. In general, typical operational pressures are maintained between about 5 to about 10 .times. 10.sup.-3 torr.
A magnetron source generally requires magnetic field strengths greater than about 100 gauss, but more typically magnetic fields in the range of 250 to 1000 gauss are used for efficient source operation. In addition, in a magnetron source, an interrelationship exists between the magnetic field strength and the cathode potential. Plasma entrapment will break down if too high a potential appears on the cathode for a fixed magnetic field strength. Similarly, for a fixed cathode voltage, if magnetic field strengths are weakened below a critical level, plasma collapse again will occur. A balance between minimum magnetic field strength and maximum cathode potential must be maintained in order to achieve stable magnetron source operation.
In the structure described by Corbani and McLeod, if the primary magnetic fields are too strong, the cathode potential, at fixed current densities, will drop, causing a loss in sputtering rate. High magnetic field strengths also produce severe local target erosion which limits the useful life of a given cathode. Generally, in order to initiate and support magnetron plasma discharge at a partial pressure of approximately 5 .times. 10.sup.-3 torr, a cathode potential exceeding -200 volts must exist. The precise cathode potential will vary, however, depending on a number of factors such as target composition and gas species.
Triode sputtering apparatus operate with considerably higher gas ionization efficiencies. This is due primarily to the use of a thermionic emitter which provides a copious supply of electrons. A large quantity of electrons, in turn, support generation of gas plasma. An increase in efficiency of generating gas plasma extends the useful operating pressure range of triode sputtering apparatus to a range lower than that of a magnetron source. A magnetron source operates at about 5 .times. 10.sup.-3 torr and a triode sputtering source operates at about 1 .times. 10.sup.-3 torr. Lowering operating and chamber pressures have a number of advantages, such as, for example, cleaner vacuum environments due to faster vacuum system pumping and higher sputter deposition efficiencies due to reduced gas scattering of the sputtered material. In addition, triode sputtering apparatus utilizes voltages of 50 volts or less to support the plasma discharge. Target voltages can be varied independently of all operational parameters. Typically, target potentials range from as low as -50 volts to as high as -2000 volts.
In prior art, triode sputtering apparatus as typified by Moseson, Gaydou and Hajzak, the function of utilizing magnetic fields is to prevent divergence of a space-oriented plasma beam, thereby increasing current flow to the target and decreasing ion bombardment of substrates and fixturing. These sputtering apparatus consist of elaborate fixturing of component parts both inside and outside an evacuable enclosure.